In mobile broadband cellular communication systems, there are several physical layer techniques that require the transmitter to be provided with knowledge of the channel response between the transmitter and receiver. Transmission techniques that make use of the channel response between the transmitter and receiver are called closed-loop transmission techniques. One example of closed-loop transmission is the use of a closed-loop transmit antenna array at the transmitter. A closed loop transmit antenna array is an array of multiple transmit antennas where the signals fed to each antenna are weighted in such a way as to control the characteristics of the transmitted signal energy according to some pre-defined optimization strategy. Generally, the closed-loop transmit antenna array weights the transmitted antenna signals based on knowledge of the space-frequency channel response between each transmit antenna and each receive antenna and attempts to optimize the characteristics of the received signal processed by the receiving device. For single antenna transmitters, the transmitter can use the knowledge of the channel to pre-equalize the channel so as to reduce or even eliminate the need for complex receive equalization at the receiver. Having knowledge of the channel response at the transmitter can also be used to select the best modulation and coding rate to use when transmitting data to the receiver.
In general, there are two methods for providing a transmitter with knowledge of the channel between each transmit antenna and each receive antenna. This discussion is focused at the downlink of a cellular system where the base station (BS) is the transmitter and a subscriber station (SS) is the receiver.
The first method is based on feedback messages from the SS, where the SS measures the channel response between the BS antennas and the SS antennas and transmits a feedback message back to the BS containing enough information that enables the BS to reconstruct the downlink channel response and perform closed loop transmission. For example, the SS could feedback a quantized version of the downlink channel estimates.
The second method is based on the reciprocity of the RF channel response in a TDD system. In a static (i.e., zero velocity) TDD system, the RF propagation channel is reciprocal, which means the downlink RF channel matrix (where the matrix refers to the channel gains between each transmit and receive antenna) at a given time-frequency point is simply the matrix transpose of the uplink RF channel matrix at the same time-frequency point. Therefore in a TDD system, a downlink channel response can sometimes be derived from an uplink data transmission if the data transmission includes pilot signals. However, in modem digital communication systems, traffic (such as web browsing) is often asymmetric, meaning that there may not be an uplink transmission associated with each downlink transmission. Or, in a broadband system a typical uplink data transmission may be of a smaller bandwidth than the typical downlink data transmission. These issues can severely degrade the performance of closed loop transmission in systems based on reciprocity. Therefore, there is a need for a closed loop transmission methods and associated signaling methods that can overcome these limitations.